title: Nitrogen deposition, plant carbon allocation, and soil microbes: changing interactions due to enrichment.
abstract: OBJECTIVE
  Nitrogen (N) inputs to the terrestrial environment have doubled worldwide during the past century. N negatively impacts plant diversity, but it is unknown why some species are more susceptible than others. While it is often assumed that competition drives species decline, N enrichment also strongly affects soil microbial communities. Can these changes affect plant-microbe interactions in ways that differentially influence success of plant species? Furthermore, can altered plant-microbe interactions lead to carbon (C) limitation in plants?
  
  
  METHODS
  We focused on a species that increases (Deschampsia cespitosa) and one that decreases (Geum rossii) in abundance in N-fertilized plots in alpine tundra. We measured soil microbes using phospholipid fatty acids, and C limitation and transfer using a (13)C tracer experiment, C:N ratios, nonstructural carbohydrates, and leaf preformation.
  
  
  RESULTS
  While N profoundly influenced microbial communities, this change occurred similarly in association with both plant species. N addition did not alter total C allocation to microbes in either species, but it changed patterns of microbial C acquisition more in Geum, specifically in gram-negative bacteria. Geum showed evidence of C limitation: it allocated less C to storage organs, had lower C:N and carbohydrate stores, and fewer preformed leaves in N plots.
  
  
  CONCLUSIONS
  Carbon limitation may explain why some species decline with N enrichment, and the decline may be due to physiological responses of plants to N rather than to altered plant-microbe interactions. Global change will alter many processes important in structuring plant communities; noncompetitive mechanisms of species decline may be more widespread than previously thought.
authors.affiliation: Department of Environmental Science, Policy & Management, University of California, Berkeley, Berkeley, California 94720, USA. ecfarrer@berkeley.edu
authors.forename: Emily C
authors.initials: EC
authors.lastname: Farrer
authors.forename: Donald J
authors.initials: DJ
authors.lastname: Herman
authors.forename: Eva
authors.initials: E
authors.lastname: Franzova
authors.forename: Trang
authors.initials: T
authors.lastname: Pham
authors.forename: Katharine N
authors.initials: KN
authors.lastname: Suding
mesh_terms: D002244:Carbon / Q000378:metabolism*; D005308:Fertilizers; D031986:Geum / Q000187:drug effects / Q000378:metabolism*; D009584:Nitrogen / Q000378:metabolism*; D058458:Nitrogen Cycle; D006109:Poaceae / Q000187:drug effects / Q000378:metabolism*; D012987:Soil; D012988:Soil Microbiology; D013045:Species Specificity
chemical_list: D005308:Fertilizers; D012987:Soil; D002244:Carbon; D009584:Nitrogen
keywords: 13C tracer; Colorado; Niwot Ridge; alpine tundra; fertilization; microbial community structure; phospholipid fatty acids; pulse-labeling